mailbox: Export mbox_flush()

The mbox_flush() function can be used by drivers that are built as
modules, so the function needs to be exported.
Reported-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>

drivers/mailbox/mailbox.c

@@ -310,6 +310,7 @@ int mbox_flush(struct mbox_chan *chan, unsigned long timeout)
 
 	return ret;
 }
+EXPORT_SYMBOL_GPL(mbox_flush);
 
 /**
  * mbox_request_channel - Request a mailbox channel.